Some nights, the job includes 75 professional instrumentalists, most of them driving many miles for rehearsals and performances, needing to be blending into one smooth-sounding unit.

Other nights, the job features less than two dozen amateurs, making music because they love it, but also needing a leader to make them sound like a unified ensemble.

For David MacKenzie, music director of the New Bedford Symphony Orchestra, as well as founder and music director of Mastersingers by the Sea, "it's the same language but a different dialect." And while his Mastersingers may not be getting paid for their work, he still demands the same musical preparation.

"We came to the agreement that we would treat the group like a professional group," he says of the chorus, which opens its seventh season next weekend with programs in Wareham and Falmouth. "They are all expected to come to rehearsals with music prepared, so that we can concentrate on music making. We focus on the details, and break things apart. Some times we spend an entire segment on eight bars of music."

Mastersingers may be made up of amateurs, but the group draws its members from singers with terrific musical experience nonetheless. "Most of them come from large choruses," MacKenzie says, "that have 90 voices or so, used to singing requiems and things like that. It took us a couple of years to find a niche in terms of repertoire, but for us it's intimate music for intimate spaces. We don't do a cappella — there are too many fine groups around that do that. And we try not to duplicate what ensembles like Sine Nomine, Gloriae Dei Cantores and the choral societies like Sippican and Chatham and Falmouth already do.

"It takes some detective work," he says, about finding the appropriate music. "I have been assembling a data base of interesting composers. It's a genre that doesn't get promoted much. But I go through the catalogues, like Schirmer, or follow tips from other conductors by word of mouth or through chat rooms."

This weekend's concerts are a microcosm of what MacKenzie looks for in terms of intimate music: early settings by Renaissance composers like Gabrielli and Schutz, cello works (featuring NBSO principal Leo Eguchi) by Vivaldi and 20th-century Spanish composer Gaspar Cassado, and a contemporary piece, "Oceanic Eyes," by Matthew Harris.

Harris' piece sets four poems by the youthful Pablo Neruda, sung here in the English translation by W.S. Merwin, using chorus, string quartet and guitar.

"This is the major work on the program," MacKenzie says. "It's what we want to do, a piece for small chorus and chamber ensemble, music that was intended for a smaller scale. Neruda was 18 or 19 years old, and the poetry has some gorgeous imagery. It's the kind of poetry where if you sit down to read it with a glass of wine you end up bawling.

"Harris' music uses a contemporary language. It's well crafted and adds incredible fluidity to the text, and the way he uses guitar — with Neruda, you have these qualities of the Spanish guitar already. There are some wonderful colors that sound like Rodrigo, or de Falla.

"It's a hard language to describe, but it's definitely accessible and effective, and it will connect with the listener. We have been exploring it from a textural standpoint, what the poetry is saying, in rehearsals, and there are so many moments that are full of dynamic possibilities.

"We have eight rehearsals for each concert," MacKenzie says, reflecting on the difference between the chorus and his professional orchestra, which has four. "So we can really get down to the level that you can't otherwise. It's an opportunity to dig deep into the expressive side of the music, and the text.

"Choral singing is the largest form of musical expression in this country," he says. "There's dozens of choruses in each city, more than any orchestra or chamber group or band. We have marine biologists, retired teachers, software designers — they all come together every Monday night to rehearse because they love the music. There is nothing more energizing or healing for me than that injection of fundamental humanity, to do what you love and to do it with love."